1. Technical Field
The invention relates to a process for producing the frame of the backrest of a motor vehicle seat.
2. Description of Related Art
Backrest frames of motor vehicle seats normally consist of metal and have the shape of an inverted U. The legs of the U are furthermore called the longitudinal bar, the base piece the transverse bar.
The backrest frame is the most strongly loaded in frontal collisions (from back or from front) of the motor vehicle when the individual occupying the seat is accelerated with the torso and head against the backrest and headrest. In this process, the longitudinal bar is flexurally loaded around an axis which lies horizontally and transversely to the direction of travel. The transverse bar is flexurally loaded around an axis which forms the intersection line between the backrest frame and a vertical frame parallel to the direction of travel. The transverse bar is torsionally loaded at the location of the headrest attachment.
The greatest bending moment occurs on the ends of the longitudinal bar on the seat surface side. With increasing distance from these ends the bending stress becomes less in the longitudinal bars.
For the purposes of lightweight construction, the metal part which forms the backrest frame at all locations should have a cross-sectional area which is matched as accurately as possible to the maximum load to be withstood. Backrest frames which are made from a round pipe or extruded section which have a constant cross sectional area over their length are therefore disadvantageous, since the minimum cross-sectional area caused by the locations of greatest possible stress is also maintained at those locations at which major stress need never be expected.
The disadvantage of the constant profile cross section is bypassed in frames which are produced as castings or from folded and bent sheet metal. However these frames cause high tool costs and a larger number of expensive production steps and quality assurance measures. Castings moreover have lower ductility, i.e. they break at low deformation.